The recent questions about restarting rocket engines in space made me wonder about this. Every mission used the OMS a couple of times at least but rendezvous missions increased the usage. The record holder is probably some multiple rendezvous mission but I don't know a good source for this information.
The OMS engines were built by Aerojet and were a member of the AJ10 engine family. They will be used on the Orion service modules (not new ones, the original ones built in the 70s (!)) and will be discarded when the service module is jettisoned.